The use of cloned DNA probes specific for the transforming regions of Moloney-MSV and Abelson-MuLV enabled us to detect homologous sequences in human DNA. These viral transforming gene-related sequences were cloned from human DNAs either by standard gene enrichment techniques or by screening cloned human DNA libraries. We have made a structural comparison of M-MSV and normal mouse cellular src and sarc sequences respectively with its human homologue by using restriction enzyme mapping and heteroduplex techniques.